1. Field of the Invention
This invention relates to a novel process for preparing a flame retardant fabric. More specifically, this invention relates to a process for impregnating by any appropriate means a fabric with a pad bath containing a buffered phosphonium salt precondensate and transferring the fabric directly to an ammoniation step without requiring an intermediate drying step.
2. Summary of the Prior Art
It is an established practice in the art to impregnate a fabric with a chemical to render the fabric flame resistant. Although numerous chemicals have been employed in this capacity, the vast majority of the chemicals contain phosphorus and nitrogen in one form or another. Typical of the phosphorus chemicals are neutralized phosphonium salts. The phosphonium salts are partially neutralized with an alkaline substance such as sodium hydroxide in the preparation of a pad bath. The fabric is passed through the bath and partially dried to a moisture content of between 5% and 20% based on the weight of the fabric. Although heat curing may be employed, the partially dried fabric is preferably exposed to gaseous ammonia or ammonium hydroxide to polymerize or cross-link the phosphorus chemical, thereby "fixing" the same in the fabric. Oxidation and process washing typically follow to yield the final fabric. For example, U.S. Pat. No. 2,772,188 discloses a process for impregnating a cellulosic textile with a flame retardant chemical by passing the textile through an aqueous pad bath containing a mixture of tetrakis-(hydroxymethyl)phosphonium chloride, tris (hydroxymethyl) phosphonium oxide and a water soluble methylolmelamine. The impregnated textile is dried at an elevated temperature and then treated with gaseous ammonia, ammonium hydroxide or a solution of ammonia in an inert solvent. Similarly, U.S. Pat. No. 2,911,322 describes air-drying prior to ammoniation.
It has been found that processes such as those of U.S. Pat. Nos. 2,772,188 and 2,911,322 suffer from a major disadvantage in that there is a tendency of the phosphonium salt condensate to migrate to the surface of the cellulosic textile on treatment with the ammonia containing solution. This creates a potential danger in that the increased concentration of the flame retardant compound on the surface of the textile may result in the fire retardant polymer being improperly formed, such as by making short chain polymers that may be easily removed in subsequent washing. Also high surface concentrations of chemicals can result in fabric stiffening. In an attempt to alleviate this problem, it has been proposed to partially "fix" the phosphonium salt in the textile or fabric prior to the ammoniation step. As disclosed in U.S. Pat. No. 3,236,676 this process involves raising the pH of the pad bath to a value in the preferred range of from 5.5 to 7.0 and heating the impregnated textile at a temperature of between 100.degree. and 180.degree. C. prior to the ammoniation step. To raise the pH value of the bath, an alkali metal hydroxide is added thereto. While this process is effective in obtaining a textile which is evenly impregnated with the flame retardant compound, the addition of the alkali metal hydroxide to the pad bath creates additional disadvantages. Specifically, if the alkali metal hydroxide is added to the bath in the form of an aqueous solution, the bath is diluted to the extent that a post-padding drying step is required. On the other hand, if the alkali metal hydroxide is added in flake form, the highly exothermic nature of the neutralization reaction causes localized heating effects which may decompose the phosphonium salts.
An alternative method of obviating the aforementioned migration problem is set forth in U.S. Pat. No. 2,983,623. As described therein, cellulosic material is made flame retardant by passing it through a pad bath, partially drying it, exposing the partially dried material to gaseous ammonia and then treating the material with an aqueous solution of ammonia. The pad bath is comprised of an aqueous solution of a phosphonium salt, such as tetrakis-(hydroxymethyl)phosphonium chloride (THPC), which has been chemically reacted with urea to form a "precondensate". In order to reduce acid degradation of the material, buffering agents, such as sodium acetate, are added to the bath in an amount sufficient to raise the bath pH to above 3.5. Exposing the dried material to gaseous ammonia ensures that the flame retardant compound becomes fixed and remains within the material, while the treatment with the aqueous solution of ammonia quickly completes the polymerization of the fixed phosphonium compound.
Another known method of rendering a cellulosic fabric flame retardant involves the wet-curing of a fabric impregnated with a melamine derivative and a phosphoric amide. As described in U.S. Pat. No. 3,632,297, the melamine derivative comprises melamine and formaldehyde or melamine, formaldehyde and an alkanol. The melamine derivative and the phosphoric amide typically are impregnated into the fabric by passing the fabric through a pad bath. The concentration of the individual coreactants in the bath and the wet pick-up are adjusted to give the desired add-ons of phosphorus and nitrogen. One manner of adjusting the wet pick-up of the fabric is disclosed as the regulation of the pressure of the squeeze rolls, which contact the fabric after it has passed through the pad bath. (See U.S. Pat. No. 3,310,419 for alternative methods of adjusting the wet pick-up of the fabric). After the padded fabric has dried to a moisture content of about 10%, either by air drying or a forced-draft oven, the fabric is maintained at a moisture content of that level and a temperature of about 20.degree. to 110.degree. C., preferably by contacting the fabric with saturated steam.
In each of the above described processes for preparing a flame retardant fabric, an integral step comprises the drying of the fabric after it has been impregnated with the flame retardant chemical. The drying step, particularly if accomplished by a forced-draft oven, may produce unwanted odors in that discrete amounts of the flame retardant chemical or other components of the treating bath are carried into the atmosphere by the evaporation of the bath solvent, e.g. water. Moreover, the drying apparatus adds considerably to the complexity of the overall process and thus makes the process more costly.
Accordingly, the search has continued for an improved process of impregnating a fabric with a fire retardant chemical which eliminates the need for a drying step prior to ammoniation. The present invention was developed as a result of this search.